Diagnostic Characteristics: About 20,000 species of flatworms exist and are characterized by a flat body, with body sizes ranging from almost microscopic to tapeworms as long as 20m. They are the simplest animals that are bilaterally symmetrical and triploblastic (composed of three fundamental cell layers). Flatworms are soft-bodied invertebrates. Unlike many other bilaterans, they do not have a body cavity (hence the term "flat").Flatworms live as either free living organisms in marine, freshwater, and terrestrial habitats, or as parasitic organisms.
There are four classes of flatworms, including Turbellaria, Monogenea, Trematoda, and Cestoidea. Turbellaria consists mostly of free living, non parasitic flatworms which live in the water, meaning that these worms do not need a host to provide it with nutrients, but can instead live on their own. There are about 4500 species . All are flat with ribbon like or leaf-like shapes. Since they lack of a respiratoy or circulatory system, they have to rely on diffusion for internal transport. http://en.wikipedia.org/wiki/Turbellaria . Monogenea and Trematoda consist of parasitic flatworms, living inside or on the surface of other animals. Monogenea parasites usually use marine hosts, while Trematoda uses vertebrates as hosts. The class Cestoidea consists of tapeworms, which are also parasites. They always live in the intestines of the host and because they don't have their own digestive systems, the absorbe the nutrients. Becuase of this, they have no llocomotive system.

Many species of flatworm are vibrantly colored. This is a Prostheceraeus, a flatworm of the class Turbellaria.

Reproduction: There are multiple ways that a flatworm can reproduce. In Turbellaria, flatworms can reproduce either asexually or sexually. Asexually, a parent worm splits into two half’s, allowing each half to regenerate into two new worms. Sexually, mates simply cross fertilize. In Cestoidea, tapeworms have long ribbons (like tails) of proglitudes, which contain sex organs and thousands of eggs. Once mature, proglitudes are released through tapeworm’s feces, and when ingested by a new organism, the eggs can grow and develop in a new host organism. Some Trematadoa, or flukes, have complicated cycles involving both humans and snails, leading to warnings not to wade in waters where these flukes are found.

Acquiring and Digesting food Most flatworms have a gastrovacular cavity with only one opening. This means that all nutrients enter and exit a flatworm from this one opening. Flatworms have only one body cavity, a gastrovacular cavity, which is even lacking in some of the smaller species. This gastrovacular cavity can be very large and complexly branched in larger flatworms because it has to move food to every part of the body. The flatworm must remain because all respiration has to be done through diffusion, and every cell has to be close enough to the outside.

Turbellaria: These worms prey on smaller animals, or feed on dead animals. This is the most primitive Platyhelminthes. Most turbellarians are very small (less than 5 mm long). Most species are marine.

Monogenea and Trematoda: These parasites attach to a host organism which provides them with all the nutrients they need to survive. They attach to either internal organs in a host, or the outer surface, for example, the skin of a fish. Monogeneans have an indirect life-cycle, meaning the always have more than one host species and th animal lives in separate hosts during different stages of its life. The host that the adult animal lives in is called the 'Primary' host, other hosts which are used by the juvenile stages are called 'Secondary' or 'Intermediate' hosts. Monogeneans normally have cephalopods (octopus and squid), fish, amphibians, reptiles and cetaceans (whales) as their primary hosts and some smaller animal that the primary host eats as their intermediate host. http://www.earthlife.net/inverts/monogenea.html#asp

Tapeworms lack any form of digestive tract, and absorb nutrients directly across their body. They live in the digestive tract of their host, where they eat predigested food from the host, absorbing it directly across their body. Tapeworms can absorb so many nutrients as to cause a nutritional deficiency in the host.

Sensing the Environment: Flatworms have a head with a pair of eye spots. These eye spots can detect light, but their main function is for smell. A flatworms nervous system is complex enough that it can learn to modify its response to a stimuli. It has a pair of ganglia, or group of nerve cells, on its head, and has two long lines of nerve cells that travel the full length of its body. The fact that one end of the flatworm is designated as the head is known as cephalization. The ganglia on the head act as a brain and specified regions are capable of detecting light, chemicals, and pressure. The platyhelminthes' nervous system is known as a simple bilateral nervous system; these organisms were the first to exhibit such a system. Some platyhelminthes contain other sense organs such as chemoreceptors, balance receptors (also known as statocysts), and rheoreceptors, which sense water movement.

Locomotion
Turbellaria: These worms move using small, hair like legs called cilia, which are on their ventral epidermis, or their bottom side of skin. Other worms can swim through the water using muscle contractions to propel them.
Monogenea, Trematoda, Cestoidea: Monogenea and Trematoda classes have suckers that allow them to attach to their hosts. Cestoidea has suckers as well, but also has hooks on its head that allow it to hold its body in place in the digestive tract of its host.

Respiration:
Flatworms don’t have specialized circulation organs because the flat shape of their body allows all of their cells to be close enough to the surrounding environment that they can simply perform respiration directly through their membrane.
This type of gas exchange is called integumentary exchange, where nurients are spread through the body in interstitial fluid (which is outside of the cells), and respiration occurs at the flatworm's integument (skin).Metabolic Waste Removal: Nitrogenous waste is removed from the body in the form of ammonia. This ammonia diffuses directly form the cells into the sounding environment. A simple excretory system, however, can also exist. This excretory system consists of a protonephridium, or a network of tubules within the mesoderm that lack internal openings. From these tubules branch a network of flame cells, which have flagella to flush out excess water and metabolic waste.Circulation: There is no specialized organ for circulation. Instead, the gastrovascular cavity is arranged in a way so that it can distribute food throughout the animal. The fluid in the body cavity may act as a hydrostatic skeleton, a site for the collection of wastes or gametes, or also as a primitive circulatory system. Platyhelominthes have their body cavities loosely filled with cells and are acoelomates.Self Protection: The parasitic flatworms have a tough covering over their bodies to help protect them. Another form of protection the worms have is if they digest hydroids, meat eating animals that resemble aquatic plants. When this occurs the worms acquire the poison and it goes to their insides for the worms to use in their own defense.Osmotic Balance: Osmotic balance is maintained by a flatworm’s simple excretory apparatus. It functions by moving fluid, with the help of ciliated (flame) cells that actually move the fluid, through branched ducts that open to the outside of the flatworm. The cilia exist on specialized cells called flame cells which are located on the tips of protonephridia, which are networks of tubules. The beating of the cilia creates preassure which forces the movement of the water. The water then exits the organism through nephridopores, an organ associated with excetion.Temperature Balance: The parasitic types of flatworms maintain a temperature of the organisms that they live in or on.
The free living types of flatworms are invertebrates, and have a body temperature that matches their surrounding temperature. If a temperature is either to cold or to hot, then the organism’s body shuts down until the temperature returns to a comfortable living environment.